What is the manipulated variable, responding variable and controlled variables in this Biuret test?

Yes, distilled water is the control variable. This determines if the solution is tainted because if it reacts to water, than it's not doing what it should do, which is to change color on contact with proteins. The control variable is the thing that you will keep the same in your experiment.

The responding variable is the dependent variable (y-axis). This is the variable that changes as a result of the changes in the manipulated variable (independent variable, x-axis).

Can you post the whole question because I can't see how I can answer what's the dependent and independent. From my understanding, the color change depends on the intensity of the protein within the chemical being tested. It's not making much sense to me.

That helps a little. Notice the glucose and distilled water are the same after the Biuret was added. This suggests that they are the control variables. Neither of these solutions contain protein, so there will not be a conclusive outcome that they contain protein.

Manipulated Variable: Amount of protein.
Responding Variable: Color change. The more purple it gets, the more peptide bonds exist, so the more protein there is.

This is the extent of what I know

Could someone explain why the biuret turned purple for the sugars? Is that supposed to happen?

According to my research, a violet color indicates the presence of proteins. Proteins give a strong Biuret reaction because they contain a large number of peptide bonds. The blue reagent changes to pink when combined with short-chain polypeptides which contain smaller numbers of peptide bonds.

That being said, shouldn't it have not turned purple for the glucose and water?

I've also researched that water plus Biuret's reagent is a negative control for the protein test. It demonstrates a negative test result (no protein present).

Egg albumin plus Biuret's reagent is a positive control for the protein test. It demonstrates a positive test result (protein is present).



Left to right: Biuret's reagent (BrR), water + BrR, egg albumin solution, egg albumin solution+ BrR.